Investigation on Zr-, Hf-, and Ta-doped submicrometric beads for DSSC photoanodes

This paper investigates the effect of Zr, Hf, and Ta doping on the performances of Dye-sensitized solar cells (DSSCs) with photoanodes comprising mesoporous hierarchical anatase spheres. Morphology, structure, and optical properties of doped and undoped materials were examined by Field Emission Scanning Electron Microscopy, X-ray diffraction, and Diffuse Reflectance Spectroscopy, and materials’ characteristics were related to DSSC fundamental photovoltaic parameters by polarization curves (under illumination and dark conditions) and external quantum efficiency measurements. Photoanodes’ charge transport and recombination properties were investigated by Electrochemical Impedance Spectroscopy. Although doping were carried out by inserting a small amount (from 0.1 to 0.3 at %) of homo and aliovalent and electron-rich (with respect to Ti4+) cation, energy conversion efficiency turned out to be lower than pure anatase photoanodes. Electron-impurity scattering phenomena in conjunction with reduced specific surface area caused by the increased bulk free energy are believed to account for the efficiency drop. However, a small beneficial effect introduced by doping on the trap-mediated recombination mechanisms has been identified.